This invention relates generally to moveable surface skimming weir devices, and more particularly, to moveable weirs for removing supernatant treated liquids from sewage and waste treatment systems.
Many of the more common sewage treatment processes involve various pretreatment steps, mixing, aeration, etc. and, ultimately, the capture of the sewage undergoing treatment in a settling basin where the biomass settles to the bottom of the basin. In the course of treating sewage a layer of solids accumulates on the top of a body of the sewage undergoing treatment. These floating solids can be composed of a biological growth which forms a cohesive layer of scum of varying thickness, which may be from several inches to a few feet in thickness. Other solids, large degradables and non-degradables such as plastic containers, etc. can also be contained with this mass together with naturally floating components such as oil and grease. The treated sewage is a clear substantially non-polluting liquid body overlaying the settled biomass upon which the scum and solids float. This body of clear, treated supernatant liquid is periodically removed from the system and the treatment cycle is repeated with input of raw sewage into the system. Such systems are referred to as variable volume activated sludge or sequencing batch reactor systems because the activated sludge constitutes the biological medium through which digestion takes place and the volume of liquid in the basis cyclically varies from a high water level to a low water level, these levels being fixed by the mechanism and mode of supernatant removal.
It is very important that the combination of scum, undigested and undigestible solids and settled solids which float on the top and the settled solids which reside in the lower portion of the liquid body be retained within the basin. when the clear supernatant liquid is withdrawn from the variable volume activated sludge basin when the clear supernatant liquid, which constitutes the treated effluent from the treatment basin, is removed. A satisfactory skimmer must be designed and constructed to permit the supernatant liquid to be removed without accepting the surface scum and debris and also without disturbing the settled solids during decantation. Efficient decantation requires that surface liquid be removed, either during the entire decantation period as is the case with commonly used skimer mechanisms or at least during the final phase of decantation when the upper surface approaches the upper interface with the settled biomass in the bottom of the basin. It is equally important that the decanter be designed and constructed to remove liquid at a relatively high rate without creating velocity vectors in the liquid flow adjacent the decanter which will cause the settled solids to mix with the clear supernatant liquid.
A variety of weirs have been designed to accomplish the removal of the treated supernatant free of contamination from the floating scum and debris and settled solids. Exemplary of such weir designs are those described by Brown and Jones in U. S. Pat. No. 4,290,887 and by the present inventor in U.S. Pat. No. 4,290,821. As the aforementioned patents describe, it is common practice to provide a weir which is permitted or caused to move downwardly during withdrawal of the treated supernatant between a top level which corresponds with the upper water level to the lower water level during decantation and to float or move to the upper water level during filling of the basin.